Neovascularization is associated with atherosclerosis and the vasa vasorum is the primary source of angiogenic vessels that supply the neovascularized area. There is evidence that vasa vasorum density increases during plaque progression, but it remains uncertain that angiogenesis has a major role in atherosclerotic plaque growth. We have shown that a truncated plasminogen activator inhibitor-1 (PAI-1) protein, rPAI-123, has significant anti-angiogenic activity. Studies performed in this funding period show that rPAI-123 inhibits fibroblast growth factor-2 (FGF2) signaling pathways and functions. We hypothesized that rPAI-123 would inhibit angiogenic vasa vasorum in atherogenic mice to result in reduced plaque progression. The hypothesis was tested in female LDLR-/-/ ApoB-100 mice that received a high fat diet for 14 weeks prior to initiating 6 weeks of rPAI-123 (n=16) or saline (n=11) treatment with continued high fat diet. Seven control animals received normal chow diet and saline treatment. The ratio of lipid area: total area in Sudan 4 stained vessels was 60% (p<0.001) less in the descending aorta and 30% (p<0.001) less in the aortic root of rPAI-123 treated animals when compared to high fat, saline treated mice. Reconstructed confocal microscopy images of CD31- probed vessels in the plaque area show that rPAI-123 reduced vessel area and length by 43 and 37% (p = 0.01), respectively when compared to high fat, saline treated controls. The left carotid artery circumference in high fat, rPAI-123 and high fat, saline groups were 24% (p=0.05) greater than the chow fed control. However, treatment with rPAI-123 reduced plaque area by 67% (p<0.001) and increased lumen area by 74% (p<0.001) when compared to the high fat, saline group. These data strongly support our hypothesis and additionally suggest that rPAI-123 promotes plaque regression. This proposal will further examine plaque regression in response to rPAI-123 in atherogenic female LDLR-/-/ ApoB-100 mice. Opposing effects of rPAI-123 on native PAI-1 functions will be studied in PAI-1-/-/ LDLR-/-/ ApoB-100. Finally, rPAI-123 binding interactions with potential candidate receptors will be investigated. Current medical treatment for atherosclerotic disease potentially prevents progression. A molecule that promotes plaque regression would provide profound medical advancement, thus making the proposed studies highly significant. Atherosclerosis is a prevalent vascular disease among Americans and is a leading cause of death. We have produced a truncated PAI-1 protein, rPAI-123, that has significant anti-angiogenic activity. Our preliminary results with rPAI-123 are novel, demonstrating for the first time that a modified PAI-1 protein can inhibit angiogenic vessels in a mouse model of atherosclerosis and promote plaque regression. These observations raise the possibility that rPAI-123 may ultimately have a therapeutic role in atherosclerosis. Current medical treatment for atherosclerotic disease potentially prevents progression, therefore, a molecule that promotes plaque regression would provide profound medical advancement, thus making the proposed studies highly significant.